The present invention relates to a plastic film with UV-barrier properties and improved transparency.
Packaging wrapping thermoplastic films are representative of the various thermoplastic films to which the present invention can apply; such films typically have a thickness below 80 xcexcm, preferably below 35 xcexcm, and are made out of at least one layer of thermoplastic material, which is extruded. They can also be oriented, in one or more direction(s) in order to achieve specific properties, for example, easy-tearing in one preferred direction. Such films are typically used for packaging purposes, for example in wrap-around packing of products, or for the making of pallets, to hold the stacked products. Such packaged products are sometimes exposed during a period of time to natural or artificial light, for example during storage, or in store""s shelves. Some of the rays constitutive of the light can alter or even degrade the structure of the packaged product. In particular, cellulose-based products are particularly sensitive to rays near the UV wavelengths, and get yellow or brown when exposed to direct or diffuse UV light.
Some films have been developed which incorporate UV-absorbing compounds, for protecting the package contents.
Some films contain inorganic compounds like metal oxides, such as for example Titanium dioxide (TiO2), or Zinc oxide (ZnO). Such films are efficient in filtering a large range of the UV rays. However, a major drawback is that TiO2 gives an opaque white color to the film. In some applications, for example when the packaged product must be visible from the outside, for example by a consumer when looking at the shelves, then such films cannot be used. A solution to limit this opacity is to have micronized compounds. Non-micronized TiO2 particles (crystals) have a grain size around 1 xcexcm, that agglomerate to even bigger sizes. This means they deliver a xe2x80x9cwhitexe2x80x9d color by scattering and reflecting the light. Micronized TiO2 has a reduced grain size of about 20 nm, which results also in smaller agglomerates. A disadvantage is: by reducing the reflection and scattering effect there are also losses in terms of UV protection since light protection is due to absorption and reflection (scattering).
Some films are colored with yellow pigments which are either added into the thermoplastic material, or at the surface of the film as a coating. Such yellow films are particularly efficient in filtering the violet part of the visible light and protecting the contents from degradation, however, some applications require that the film be transparent and not colored, for example when packaging colored products, or food: in such cases, the consumer must be able to see the real color of the product.
Some films have been developed which feature UV-absorbing properties, and have a transparent color. Such transparent films are achieved by using polar organic compounds which have UV-absorbing properties while remaining transparent when added to a plastic film composition. However, a major drawback of such organic compounds is their low stability in an apolar thermoplastic base resin. In other words, such organic compounds are very likely to migrate into a homogeneous thermoplastic layer, so that the film shows a greasy surface and loses some of its UV-absorbing properties, which is clearly undesirable to the consumer. The solution which is generally used to counterbalance this poor stability is to make films which comprise several layers. Typically, the layer comprising the organic UV-absorbing compound is made of polar thermoplastics such as PET (polyethylene terephtalate) or PEN (polyethylene naphtalate) and sandwiched between two other layers so that said organic compound cannot escape, because organic compounds are polar and PET/PEN are apolar compounds. In this way, the film is more likely to keep its UV-absorbing properties. Such films are however quite expensive to manufacture, due to the complex process to make the several layers.
As for organic compounds, it has been shown that benzotriazoles are efficient for stopping rays whose wavelength is comprised within the range of 300 to 370 nm, while benzophenones are efficient to stop rays with a wavelength comprised within 300 to 400 nm. As for inorganic compounds, micronized metal oxides (TiO2/ZnO) prove to be efficient against UV rays in a wavelength range comprised between 200 and 320 nm. White cellulose-based products show degradation and yellowing when exposed to rays whose wavelength is typically comprised within the UV range.
One main object of the present invention is to provide a thermoplastic film with improved UV-absorbing properties, with a low thickness suitable for the purpose of packaging, which is transparent, non-colored and clear, which is inexpensive and easy to manufacture, and whose UV-absorbing properties are substantially constant over a long period of time.
The present invention is directed to a film made out of a thermoplastic material with UV-absorbing properties, characterized in that its composition further comprises a combination of at least one UV-absorbent organic compound with at least one inorganic UV-absorbent compound, for improved barrier against UV rays. Preferably, the organic compound is a benzotriazole, and the inorganic compound is micronized titanium dioxide. More preferably, the film""s material further comprises a binder compound, for example a benzophenone, in order to improve the compatibility between the organic component and the base-resin. Preferably, the film thickness is below 80 xcexcm, more preferably below 35 xcexcm.